Motor control system with compensation for low-frequency variations in motor energizing voltage

ABSTRACT

A phase-shift stabilizer is connected in series with an inverter to provide a selective phase-shift of the individual timing pulses passed to the inverter, while maintaining a constant average frequency of these timing pulses. A rectifier circuit in a feedback line from the inverter output side provides a control voltage signal which modifies the phase-shift of each timing pulse in a direction and by an amount appropriate to offset undesired low-frequency fluctuation in the average output voltage from the inverter. The phase-shift stabilizer includes a sawtooth generator for producing a constant amplitude output signal varying in frequency as a function of a frequency regulating signal. A filter is connected to provide a signal which is a function of the undesired low-frequency modulation of the inverter output. A comparator circuit is connected to receive a composite signal formed by mixing the filter output and sawtooth generator signals, and to compare the composite signal against a reference signal. The comparator provides a phase-shifted output signal for controlling the inverter frequency to compensate for the unwanted low-frequency modulation.

United States Patent Bejach 1 MOTOR CONTROL SYSTEM WITH COMPENSATION FORLOW- [151 3,670,235 [4 1 June 13, 1972 FOREIGN PATENTS OR APPLICATIONS1,148,439 4/1969 Great Britain ..32l/l8 FREQUENCY VARIATIONS IN MOTOR PW l h ENERGIZING VOLTAGE nmary ExammerlI lam H. Be a, .Ir. IAttorney-Donald W. Banner, William S. McCurry and John [72] Inventor:Benton Bejach, Santa Ana, Calif. w, B h [73] Assignee: Borg-WarnerCorporation, Chicago, Ill. [57] ABSTRACT [22] Filed: Sept 1970 Aphase-shift stabilizer is connected in series with an inverter- [21]Appl. No.: 75,472 to provide a selective phase-shift of the individualtiming pulses passed to the inverter, while maintaining a constantaverage frequency of these timing pulses. A rectifier circuit in if:63:63 a feedback line from the inverter output side provides a cond 230trol voltage signal which modifies the phase-shift of each tim- 1 e o m307/228 ing pulse in a direction and by an amount appropriate to offsetundesired low-frequency fluctuation in the average output voltage fromthe inverter. The phase-shift stabilizer includes a [56] ReferencesCited sawtooth generator for producing a constant amplitude outputsignal varying in frequency as a function of a frequency regu- UNITEDSTATES PATENTS lating signal. A filter is connected to provide a signalwhich is a I function of the undesired low-frequency modulation of thein- 1418527 5 g z verter output. A comparator circuit is connected toreceive a 9 g 7/1967 318/231 composite signal formed by mixing thefilter output and saw- 333190 6/1970 "5 X I tooth generator signals, andto compare the composite signal 3 9/1967 321/2 x against a referencesignal. The comparator provides a phase- 5 52 lo/1964 fil ""307/229 1shifted output signal for controlling the inverter frequency to l 0 ancompensate for the unwanted low-frequency modulation.

9 Claims, 16 Drawing Figures F A-C requency Regu Icmng Rectifier S i qno I Output '4 I5 20 s I8 I2 1 IO Phose Shift l6 II at r Osc l 0 Stab. 1Drlver ulnverter Motor I7 A 2! l9 v 22 Control Voltage 123 S' RectifierP'ATENTEDJIIIIIBIIII? 3,670,235

SHEEI 10F 4 FIG. 1 .3

' A-C Frequency Regulanng RectItIer Signal Input Output l5 V 20 s'gnoll8 I2 r H Io PhaseShift l6 OscIllator Stab. L DrIver Inverter 22 ControlVoltage 23 I s'gnol Rectifier 26 Freq. Signal From Osc.

I I l I I I I I I I H FIGQA TIme I I I I I I I IIIIII IFIGQB ,l) 23Output SIgnal From 27 Phase Shift Stab.

45 35 Low Frequency 46 23 SIgnal I Actlve 36 38 om 0 T Control Filter 40p w (W VQHOge 1 4I Signal SIgna| 43 F Clipper Com osite Si not Feedback4 p 9 3o Sawtooth 3 Ref.

Sawtooth slgnol 33 Generator I 32 Inventor RFrequenCy Benton Be ach equatIng Q Signal v \A-a\/ )4 Atto ney PATENIEDJuu 1 3 I972 {11. 670,235

' \94 FIG. 7/\

FIGJC new Inventor i FIGJG Benton Bejuch MOTOR CONTROL SYSTEM WITHCOMPENSATION FOR LOW-FREQUENCY VARIATIONS IN MOTOR ENERGIZING VOLTAGEBACKGROUND OF THE INVENTION c energizing voltage for the motor. Theinverter can be energized from any suitable d-c source, such as arectifier circuit, one or more batteries, a fuel cell, or otherarrangement. The oscillator may be a precise arrangement such as avoltage-controlled oscillator (VCO) in which a carefully regulated d-cvoltage is utilized to govern the frequency of the oscillator timingpulses. These pulses may be fed directly or through a driver stage tothe inverter to regulate the frequency of firing the SCRs or otherswitches in the inverter, and thus correspondingly effect a regulationof the inverter output voltage to produce a concomitant variation in themotor speed.

One problem that arises with such systems is the occasional presence ofan unwanted low-frequency modulation signal on the inverter a-c outputvoltage utilized to energize the a-c motor, whether synchronous,synchronous reluctance, or an induction motor. Such modulation orcontinuous oscillations, most common at low inverter frequencies (forexample, up to ten Hertz), is generally the result of inverter-motorinteraction, although the motor alone can give rise to suchoscillations. In addition a sudden increase in the load on an a-c motormay produce a momentary instability which is reflected back as thisundesired low-frequency modulation.

One significant advance in the correction of the total system operationto compensate this unwanted modulation is described and claimed in U.S.Pat. No. 3,482,157-Brden et al., which issued Dec. 2, 1969, and isassigned to the assignee of this invention. As there described ademodulator unit senses the unwanted aberration in the inverter a-coutput voltage, and inserts an offset or correcting signal directly intothe oscillator which provides the timing pulses for controlling thefrequency of inverter operation. Although substantial intprovement wasnoted, there was still some disturbance of the additional motors whenanother motor was line started.

Moreover the described system does not admit of ready incorporation witha system in which the oscillator is a fixed or digitally-controlledfrequency source. It was also noted that some speed errors, not similarto the average type speed error but rather long tailed transient errors,were probably derived from the bandpass filter arrangement utilized inthat system to effect the low-frequency compensation. This may result insome speed error in the a-c motor(s) being controlled by the system.

It is therefore a primary consideration of the present invention toprovide an a-c motor control system with low-frequency compensation suchthat a line start of a single motor will minimize disturbances to thealready-operating system.

Another important consideration of this invention is the provision ofsuch a system which can readily be utilized with oscillators of thefixed frequency or digitally-controlled types to provide precise speedcontrol of the motor( s). Another important consideration is theprovision of a motor speed control system in which the desiredcompensation is effected while insuring that the average frequency ofthe timing pulses supplied to the inverter is precisely the same as theaverage frequency of the basic timing pulses derived from a preciselycontrolled oscillator or frequency source.

SUMMARY OF THE INVENTION The control system of the present invention isuseful with an arrangement in which an inverter provides an ac voltagefor energizing an a-c motor. Such systems normally utilize an oscillatorfor supplying a frequency regulating signal, such as timing pulses, toregulate the inverter operation. This in turn regulates the frequency ofthe inverter a-c output voltage to control the a-c motor speed.

Particularly in accordance with the present invention, a phase-shiftstabilizer is coupled in series between the oscillator and the inverter.The stabilizer includes circuit means for effecting a controlledphase-shift of the individual timing pulses received from theoscillator, while maintaining constant the average frequency of theoutput pulse signals supplied to control the inverter operation.Additional circuit means, such as a rectifier or a current-sensingcircuit, is coupled between the inverter and the phase-shift stabilizerto provide a control voltage signal denoting low-frequency variationsin'the inverter ac output voltage. This control voltage signal is usedto regulate the extent of the selective phase-shift of the individualtiming pulses to compensate for the undesired low-frequency oscillationsin the inverter-motor system.

THE DRAWINGS In the several figures of the drawings like referencenumerals identify like elements, and in the drawings:

FIG. 1 is a block diagram of a motor control system in which a system ofthe present invention has been incorporated;

FIGS. 2A and 2B are illustrative showings useful in understanding theoperation of the invention;

FIG. 3 is a block diagram, partly in schematic form, illustrating majorcomponents in a preferred embodiment of the invention;

FIGS. 4 and 6 are schematic diagrams which taken together supply thecircuit details of the system shown more generally in FIG. 3;

FIG. 5 is a partial schematic diagram depicting an alternate circuitarrangement for a portion of the system shown in FIG.

FIGS. 7A-7G are graphical illustrations useful in understanding theoperation of the present invention; and

FIGS. 8 and 9 are diagrams of circuits suitable for use with the systemof this invention.

GENERAL SYSTEM DESCRIPTION In FIG. I an a-c motor 10 is shown energizedby the a-c output voltage received over line 11 from an inverter 12. Arectifier circuit 13 receives a-c energy and supplies a d-c energizingvoltage over line 14 to the inverter. The frequency of inverteroperation, and thus the frequency of its a-c output voltage, isregulated by a train of timing pulses provided by an oscillator stage15. This oscillator can be of the voltage controlled type (VCO), whichreceives a d-c voltage from a potentiometer 16 to regulate the frequencyof the oscillator timing pulses which appear on output line 17 as thefrequency regulating signal for the system. In general such signals maybe passed over a driver or amplifier stage 18, thus providing controlpulses on the line 19 to regulate the switching times of the SCR's (orother switches in the inverter) and thus regulate the speed of motor 10.

In accordance with the present invention a phase-shift stabilizer 20 iscoupled in series between the line 17 from the oscillator and the inputline 21 to the driver stage. This stabilizer circuit produces aselective, controlled phase-shift with respect to each of the timingpulses in the frequency regulated signal, to provide an output signal online 21 in which the average frequency of the pulses is exactly the sameas the average frequency of the oscillator timing pulses. The extent ofeach pulse displacement is determined by a control voltage signal, whichmay be derived from the output side of inverter 12. A portion of theinverter a-c output voltage is passed through a rectifier circuit 22 toprovide the control voltage A The frequency regulating signal on line 17may contain a train of timing pulses such as the pulses 25 in FIG. 2A,which are spaced apart by the same time intervals. Each pulse signal 25appears precisely in the center of a window or time interval 26 whichcorresponds to the period of oscillator operation. After the phase shiftinstabilizer 20, the output signal pulses 27 appear as shown in FIG. 28.Again each individual pulse appears within a window or time period asrepresented by bracket 28, but where necessary to compensate for theundesired Iow-frequency modulation of the a-c output voltage, each pulse27 is shifted from the center of its respective window 28. However, itis emphasized that the average frequency of the output pulses 27 ismaintained exactly the same as that of the input timing pulses 25, sothat a precise and accurate motor speed regulation is still maintainedin exact accordance with the setting of the frequency regulating signal.

DETAILED DESCRIPTION OF THE INVENTION FIG. 3 illustrates major circuitcomponents of the phaseshift stabilizer 20. As there shown the frequencyregulating signal is applied over line 17 to a sawtooth generatorcircuit 30, and a feedback line 31 assists in maintaining a constantamplitude of the sawtooth signal on line 32. The frequency of thissawtooth signal is a direct function of the frequency of the signal online 23. A reference signal is supplied from the potentiometer 33 over aline 34 to another input connection of the sawtooth generator circuit.

The control voltage signal is applied over line 23 to a filter 35, whichin a preferred embodiment is an active filter. Filter 35 removes thehigh-frequency ripple and leaves only any undesired'low-frequencymodulating signal on line 36. A clipper stage 37 is connected to line 36so that the resultant signal on line 38 is amplitude controlled, andthis clipped signal is passed over a resistor 40 to terminal 41. Thesawtooth voltage signal is applied over resistor 42 to the sameterminal. Thus the clipped low-frequency signal is mixed with thesawtooth signal to provide a composite signal on line 43 for applicationto one input connection of a comparator stage 44. The other inputconnection of comparator stage 44 receives a reference signal over line45 from a reference potentiometer 46. The

. output signal from comparator stage 44 is passed over line 21 tocontrol the frequency of inverter operation. The mixing of the undesiredlow-frequency signal with the sawtooth voltage signal to provide acomposite signal for application to the comparator is an important partof the inventive system, contributing to'the requisite compensation ofthe inverter operating frequency to offset the undesired low-frequencymodulation which would otherwise occur.

FIG. 4 depicts the details of the sawtooth generatorcircuit 30. Thiscircuit includes first and second operational amplifiers 50 and 51, atransistor 52 coupled to the output side of op amp 51 between a feedbackloop for this op amp and a charging capacitor 53 across which thesawtooth voltage waveform is developed. This circuit also includes ashorting transistor 54 for effecting discharge of capacitor 53 andrestarting the cycle, and a pair of emitter-follower coupled transistors55, 56. The circuit is energized by applying a voltage positive onconductor 57 relative to the potential on the reference conductor 58. AZener diode 60 is coupled between reference conductor 58 and anintermediate conductor 61 to provide a potential difi'erence betweenconductors 57 and 61. In general the interconnection and operation ofsawtooth generators are well known and no specific circuit discussion isrequisite for those skilled in the art. To minimize the need forexperimentation and complete a thorough teaching of this invention, atable of typical circuit component identifications and values will beset out hereinafter.

In general a sawtooth voltage waveform is developed across capacitor 53,as a function of the output signal from op amp 51 and the conductionlevel of NPN type transistor 52. The

connection of transistor 54. Thus the precise circuit and manner ofconnection to pass the frequency information from the basic oscillatorstage to the sawtooth generator circuit 30 may be varied.

In FIG. 4 op amp 51 is a linear amplifier and has a feedback 5 loop,including transistor 52 and conductor 70, to provide an output orcharging currentfor capacitor 53 which is extremely linear. Thus thesawtooth voltage waveform from the capacitor 53 is applied over theemitter-coupled stages 55, 56 and then passed over conductor 32 toresistor 42 in the circuit of FIG. 6. The emitter-coupled stages 55, 56(FIG. 4) offer a high input impedance and thus avoid loading of thecapacitor 53. No significant current is passed over line 32 which onlyprovides the output sawtooth voltage waveform signal to the otherportions of the circuit. It is noted that the same sawtooth waveformsignal is applied over feedback line 31 and resistors 71, 72 to theupper input connection of op amp 50. The other input connection of thisop amp is coupled over a resistor 73 and a conductor 34 to the movablearm of reference potentiometer 33. Thus this other op amp 50 connectedin the feedback loop for op amp 51 has the effect of accelerating theamount of charging current passed into the charging capacitor 7 53 asthe frequency of operation rises,'to maintain the amplitude of thesawtooth voltage waveform substantially constant. By increasing thecharge rate at higher frequencies and decreasing the charge rate atlower frequencies, this constant amplitude of the output voltagewaveform is readily maintained. By way of example, the charge currentmight be of the order of 10 milliamps at a given high frequency, andatonetenth of this high frequency, the amount of the charging currentwould be reduced to l milliamp. 1

FIG. 6 depicts circuit details of the active filter 35, clipper 37 andcomparator circuit 44. Although a two-pole active bandpass filter isillustrated, it will be appreciated that other types of filters can beselected to remove the extraneous noise and provide only thelow-frequency modulation signal on the output line 36 of the filter.Considering the output signal from the inverter 12 (FIG. 1),rectification of this a-c signal in stage 22 produces an output signalsuch as that referenced 94 in FIG. 7A. This illustration is that of arectified signal with no undesired low-frequency modulation. When theselowfrequency variations, in the order of from 2 to 10 Hertz, arepresent, the rectified waveform is similar to that referenced 95 in FIG73. However, it is desirable to remove the ripple or high-frequencynoise and leave only the low-frequency signal depicted by the waveform96 in FIG. 7C. This is the function of the active filter 35 includingthe two op amps 100, 101 in FIG. 6. As there shown, the rectified signalsuch as 95 in FIG. 7B is applied over input conductors 23A, .238 to theinput connections of the first op amp 100. The output signal from thisstage is applied through the second op amp 101, and the output signal ofthe active filter unit 35 is then passed over resistor 102 and conductor36 to the common line 38. The interconnection and operation of suchactive filters are now well known and understood and need not bedescribed herein. However a table of components will be set out at theend of the specification to facilitate practice of the invention with aminimum of experimentation.

Clipper stage 37 includes a pair of Zener diodes 103, 104,

connected back-to-back between common line 38 and reference line 105.This circuit is arranged to remove transients and other unwanted highvoltage signals from the low-frequency signal (FIG. 7C) which is presentat theoutput side of the active filter. For example if each of thediodes 103,

104 is rated at 7.5 volts, then the total swing or amplitude or minus7.5 volts from the design center voltage. This operation is importantbecause a line voltage transient could provide a very large transientthrough the active filter and possibly cause a disparity of the 1:1correspondence between the pulses in the output signal on line 21 andthe frequency of the input pulses on line 17.

Thus the signal represented by waveform 96 in FIG. 7C is applied overresistor 40 to the junction 41 in FIG. 6, and the sawtooth signaldepicted by waveform 97 in FIG. 7D is applied over line 32 and resistor42 to the same junction 41. Thus the signal applied over conductor 43 tothe lower input connection of the op amp 44 in the comparator circuit isa composite of the low-frequency and sawtooth signals, and thiscomposite signal is represented by the waveform 98 in FIG. 7B. The otherinput connection of op amp 44 is coupled over a resistor 106 andconductor 45 to the movable arm of a reference potentiometer 46. Thuswhenever the voltage of the compositesignal on line 43 reaches thereference voltage level (broken line 99 in FIG. 7E) applied to the upperinput connection of op amp 44, the device changes state in accordancewith well known operating principles. That is, the device will saturateand the output signal will change state. The output signal isrepresented by the waveform 107 in FIG. 7F, and is produced at theoutput connection 6 of op amp 44.

Considering the waveform 107, it is manifest that whenever the sawtoothwaveform is reset or goes to zero as capacitor 53 (FIG. 4) isdischarged, the negative-going or trailing edges of the pulses inwaveform 107 are produced. The positive-going or leading edges of thepulses are generated at the times when the ramp portion of the compositesignal 98 reaches the reference level 99 established by the setting ofpotentiometer 46.

The output signal from op amp 44 is passed through coupling capacitor108 and resistor 1 10 to the base of an NPN type transistor 111. Thecollector of this transistor is coupled to the lower end of the primarywinding 112 of an output transformer 113, which includes a secondarywinding 114 connected to supply the output signal or timing pulses overconductors 21A, 21B. The emitter of transistor 1 l 1 is coupled toconductor 61. Accordingly the negative-going or trailing edges of thepulses in FIG. 7F do not affect the output circuit. However, each timethe leading edge or positive-going portion of a pulse is applied throughcapacitor 108 and resistor 110, transistor 1 11 is rapidly driven intosaturation and current flows through the primary winding of transformer113 to pass an output timing pulse over conductors 21A, 21B. Thesepulses are referenced 1 15 in FIG. 70. It is thus apparent that theindividual pulses 115 are displaced in phase or in time by an amount andin a sense appropriate to compensate the undesired low-frequencymodulation, such as that depicted by the waveform 96 in FIG. 7C.

The circuit illustrated in FIGS. 4 and 6 was energized from a voltagevolts positive, applied over conductor 57, relative to the reference orground potential on conductor 58. The control voltage for applicationover conductors 23A, 23B was simply derived from a rectifier bridgecircuit coupled in the system as shown generally in FIG. 1. Of course, avoltage di-' vider arrangement can be utilized to insure that theamplitude of the resultant signal on line 23 is appropriate to the lowlevel of the other signals in the illustrated control system. Solely toassist those skilled in the art to make and use this invention with aminimum of experimentation, and in no sense by way of limitation, atable of component identification and values is set out below.

77,80,86,143 1.5 K Resistance 82,102 2 K values in 3.3 K ohms 5% 147 4.3K V4 watt 33,46,135,136 5 K 63, 137 5.1 K 81 6.8 K 42,88,140 10 K106,130,141 15 K 40,122 24 K 138 27 K 152 47 K 125,134 51 K 64,71,72,73,

128,150 100 K 124 200 K 74 1 M 53 0.047 microfarad 75 1.0 microfarad76,85 4700 picofarads 78,87,148 100 picofarads 108 0.0022 microfarad 1233.0 microfarads 126,133 0.1 microfarad 127 33 microfarads 1 19 78microfarads FIG. 8 depicts a rectifier arrangement suitable forproviding the control voltage signal for application to the phase-shiftstabilizer. As there shown, three conductors 160, 161, and 162 arecoupled to the output conductors (not shown) between the inverter 12 andthe motor 10. Of course, if a single phase inverter and motor drivesystem is utilized, only two conductors are provided. In FIG. 8 thediodes in rectifier bridge 22 rectify the a-c signal received overconductors 160, 162 to provide an output d-c voltage signal betweenconductors 163 and 164. A gain control potentiometer 165 and a resistor166 are coupled in series between conductors 163, 164. Output conductor23A is coupled to conductor 163, and the other output conductor 23B iscoupled to the common connection between potentiometer 165 and resistor166. Thus the control voltage signal is present between conductors 23A,23B, and the system gain can be adjusted by varying the setting ofpotentiometer 165. In some systems it may not be necessary to provide again control, and for such systems the potentiometer 165 can be replacedby a simple resistor.

FIG. 9 depicts another arrangement for providing the control voltagesignal. As there shown the d-c bus current from rectifier 13 is passedover the main bus conductors 14a and 14b to the inverter 12. A droppingresistor 170, of only a few ohms resistance, is connected in the mainline 14b. Ac-

cordingly the output signal on lines 23A, 23B provides a control voltagesignal reflecting any low-frequency variations in the inverter d-c buscurrent. Presently the arrangements of FIGS. 8 and 9 have proven themost suitable for deriving an effective indication of the undesiredlow-frequency oscillations. There appears to be an interactiveoscillation between the inverter and motor in many systems, and evenmotors with zero source impedance can oscillate at an undesired lowfrequency. However, the illustrated systems have proven effective tosubstantially eliminate the low-frequency oscillations without in anysense diminishing the precise frequency control in such systems.

While only a particular embodiment of the invention has been describedand illustrated it is manifest that various modifications andalterations may be made therein. It is there fore the intention in theappended claims to cover all such modifications and alterations as mayfall within the true spirit and scope of the invention.

I claim:

1. In a motor control system having an inverter for providing an a -cvoltage, and means for supplying a frequency controlling signalincluding spaced timing pulses to regulate the frequency of inverteroperation and correspondingly regulate the frequency of the inverter a-coutput voltage, the improvement which comprises:

a phase shift stabilizer, coupled in series with the inverter, includingcircuit means for effecting a controlled phase shift of individualtiming pulses while maintaining constant the average frequency of thephase-shifted timing pulses in the output signal supplied for regulatingthe inverter, and circuit means, coupled to the inverter and to thephase shift stabilizer, for providing a control voltage signal whichvaries as a function of undesired lowfrequency variations, to effect aselective phase shift of the individual timing pulses as a function ofsuch undesired low-frequency variations, in which said phase shiftstabilizer comprises a filter, connected to receive said control voltagesignal and to provide a low-frequency signal which varies with thelow-frequency variations of the inverter a-c output voltage, a sawtoothgenerator, connected to receive the frequency-controlling signal and toprovide a sawtooth voltage output signal of substantially constantamplitude and of a frequency determined by the timing pulses in thefrequency-controlling signal,

and a comparator stage, having a first input terminal connected toreceive a reference signal, a second input terminal connected to receivea composite signal formed by mixing the low-frequency signal and thesawtooth voltage output signal, and an output terminal, connected topass phase-shifted timing pulse signals toward the inverter tocompensate for the low-frequency variations of the inverter a-c outputvoltage.

2. In a control system for energizing-an a-c motor from an inverter,with an oscillator circuit for supplying a frequencyregulating signalincluding a train of timing pulses to control the frequency of inverteroperation and correspondingly regulate the frequency of the inverter a-coutput voltage, the improvement which comprises a phase shift stabilizercircuit, including:

circuit means, coupled to the inverter, for providing a control voltagesignal including a high-frequency component related to the frequency ofinverter operation, and which may include an undesired low-frequencycomponent;

a filter circuit, connected to receive the control voltage signal fromthe circuit means, for removing the highfrequency component andproviding an output signal ineluding only the low-frequency component;

a sawtooth generator circuit, connected to receive thefrequency-regulating signal and provide a sawtooth output voltage signalsynchronized in frequency with the timing pulses in thefrequency-regulating signal, including a feedback circuit for assistingin regulating operation of the sawtooth generator to provide asubstantially constant amplitude output signal; and

a comparator circuit, including a first input terminal connected toreceive a reference signal, a second input terminal connected to receivea composite signal provided clipper circuit comprises a pair of diodesconnected back-to back between the output side of the filter circuit anda plane of potential corresponding to the design center potential forthe clipper circuit.

5. A control system as claimed in claim 2 in which the filter circuit isan active filter circuit, including a first operational amplifierconnected to receive the control voltage signal at its input terminals,and a second operational amplifier connected to receive a referencesignal at one of its input terminals and the output signal from thefirst operational amplifier at its other input terminal.

6. A control system as claimed in claim 2 in which the sawtoothgenerator circuit includes a charging capacitor, a first operationalamplifier and a transistor connected to provide a substantially constantcharging current for the chargingcapacitor, a discharge transistorcoupled in parallel with the charging capacitor for rapidly completing adischarge path in response to receipt of a timing pulse from theoscillator circuit, and at least one emitter-coupled transistor coupledbetween the charging capacitor and the comparator circuit to supply thesawtooth output voltage signal for mixing with the clipped low-frequencysignal.

7. A control system as claimed in claim 6 and in which said sawtoothgenerator circuit further comprises a feedback circuit, including afeedback conductor coupled to the emittercoupled transistor, and asecond operational amplifier coupled between the feedback conductor andthe first operational amplifier to assist in maintaining a substantiallyconstant amplitude of the sawtooth output voltage signal.

8. A motor control system as claimed in claim 2, in which said circuitmeans comprises a rectifier circuit, coupled between the output side ofthe inverter and the filter circuit of the phase-shift stabilizer, forproviding the control voltage signal.

9. A motor control system as claimed in claim 2, in which said circuitmeans includes a resistor, coupled in series with the main d-c bussupplying energy to said inverter, for providing said control voltagesignal as a function of undesired lowfrequency variations in the d-c buscurrent.

1. In a motor control system having an inverter for providing an a-cvoltage, and means for supplying a frequency controlling signalincluding spaced timing pulses to regulate the frequency of inverteroperation and correspondingly regulate the frequency of the inverter a-coutput voltage, the improvement which comprises: a phase shiftstabilizer, coupled in series with the inverter, including circuit meansfor effecting a controlled phase shift of individual timing pulses whilemaintaining constant the average frequency of the phase-shifted timingpulses in the output signal supplied for regulating the inverter, andcircuit means, coupled to the inverter and to the phase shiftstabilizer, for providing a control voltage signal which varies as afunction of undesired low-frequency variations, to effect a selectivephase shift of the individual timing pulses as a function of suchundesired low-frequency variations, in which said phase shift stabilizercomprises a filter, connected to receive said control voltage signal andto provide a lowfrequency signal which varies with the low-frequencyvariations of the inverter a-c output voltage, a sawtooth generator,connected to receive the frequency-controlling signal and to provide asawtooth voltage output signal of substantially constant amplitude andof a frequency determined by the timing pulses in thefrequency-controlling signal, and a comparator stage, having a firstinput terminal connected to receive a reference signal, a second inputterminal connected to receive a composite signal formed by mixing thelow-frequency signal and the sawtooth voltage output signal, and anoutput terminal, connected to pass phase-shifted timing pulse signalstoward the inverter to compensate for the low-frequency variations ofthe inverter a-c output voltage.
 2. In a control system for energizingan a-c motor from an inverter, with an oscillator circuit for supplyinga frequency-regulating signal including a train of timing pulses tocontrol the frequency of inverter operation and correspondingly regulatethe frequency of the inverter a-c output voltage, the improvement whichcomprises a phase shift stabilizer circuit, including: circuit means,coupled to the inverter, for providing a control voltage signalincluding a high-frequency component related to the frequency ofinverter operation, and which may include an undesired low-frequencycomponent; a filter circuit, connected to receive the control voltagesignal from the circuit means, for removing the high-frequency componentand providing an output signal including only the low-frequencycomponent; a sawtooth generator circuit, connected to receive thefrequency-regulating signal and provide a sawtooth output voltage signalsynchronized in frequency with the timing pulses in thefrequency-regulating signal, including a feedback circuit for assistingin regulating operation of the sawtooth generator to provide asubstantially constant amplitude output signal; and a comparatorcircuit, including a first input terminal connected to receive areference signal, a second input terminal connected to receive acomposite signal provided by mixing the sawtooth output voltage signalwith the clipped low-frequency signal, and an output terminal forproviding timing pulse signals of constant average frequency but whichare individually phase-shifted to offset the undesired low-frequencymodulation of the inverter a-c output voltage.
 3. A control system asclaimed in claim 2 and further comprising a clipper circuit, coupled tothe filter circuit, for receiving the low-frequency signal and limitingthe positive-going and negative-going peak excursions of thelow-frequency signal.
 4. A control system as claimed in claim 3 in whichsaid clipper circuit comprises a pair of diodes connected back-to-backbetween the output side of the filter circuit and a plane of potentialcorresponding to the design center potential for the clipper circuit. 5.A control system as claimed in claim 2 in which the filter circuit is anactive filter circuit, including a first operational amplifier connectedto receive the control voltage signal at its input terminals, and asecond operational amplifier connected to receive a reference signal atone of its input terminals and the output signal from the firstoperational amplifier at its other input terminal.
 6. A control systemas claimed in claim 2 in which the sawtooth generator circuit includes acharging capacitor, a first operational amplifier and a transistorconnected to provide a substantially constant charging current for thecharging capacitor, a discharge transistor coupled in parallel with thecharging capacitor for rapidly completing a discharge path in responseto receipt of a timing pulse from the oscillator circuit, and at leastone emitter-coupled transistor coupled between the charging capacitorand the comparator circuit to supply the sawtooth output voltage signalfor mixing with the clipped low-frequency signal.
 7. A control system asclaimed in claim 6 and in which said sawtooth generator circuit furthercomprises a feedback circuit, including a feedback conductor coupled tothe emitter-coupled transistor, and a second operational amplifiercoupled between the feedback conductor and the first operationalamplifier to assist in maintaining a substantially constant amplitude ofthe sawtooth output voltage signal.
 8. A motor control system as claimedin claim 2, in which said circuit means comprises a rectifier circuit,coupled between the output side of the inverter and the filter circuitof the phase-shift stabilizer, for providing the control voltage signal.9. A motor control system as claimed in claim 2, in which said circuitmeans includes a resistor, coupled in series with the main d-c bussupplying energy to said inverter, for providing said control voltagesignal as a function of undesired low-frequency variations in the d-cbus current.